The rear cover on the 7.25 is hexagonal looking and has axle tubes that are 2.5" in diameter. The 8.25 rear cover is rounded with a slight oval shape and has 3.5" diameter axle tubes.
The size of the brakes referes to the inside diameter of the drum that the shoe contact. On the outside of the drum near the wheel stud will be the service limit dimension the drum can be macjhined to. It will be 9.***" for 9" brakes and 10.***" for 10" brakes.
If your car is a \6 and has drums all the way around the brakes will be 9" front and rear. The car will also have a 7.25" axle.
If your car is a 318 with front disks the rear drums will be 10". IT may or may not have the 8.25" rear end.
If your car is a 360 it will have the 8.25"
Disclaimer: As with all things mopar there is always an exception to every rule. So the only way for sure to know is to look.
